The studies on the epithelial cell types in the adult rat mammary gland and their evolution will be completed. The study of markers for various cell types will be continued; emphasis will be placed mostly on monoclonal antibody to various types of rat mammary cells. The types of cells involved in carcinogenesis by N-nitrosomethylurea will be defined on the basis of their developmental history and potential, and of available markers. The condition leading to dome formation in a spontaneously doming mammary cell line will be studied and possible effectors identified. The role of the Thy-1 antigen on dome formation will be investigated further.